Dear Editor

We have prepared a revised manuscript to address the PROBLEMS WITH MANUSCRIPT and comments of the referees.

Here it is the list of changes to the manuscript:


* Captions of figures 1 to 4: added the "(Color online)" at the beginning of every caption.

* The title was changed from:
"Direct generation of an exponentially rising single photon field from parametric conversion in atoms"
to:
"Generation of an exponentially rising single photon field from parametric conversion in atoms"

* The introduction of the homodyne measurement (pag. 2, second column of the manuscript) has been modified from:
"We now proceed to determine the temporal envelope of the heralded single
photon fields by measuring the field quadrature in the time domain~\cite{Yuen:83,lvovsky:2001} via a balanced homodyne detection."
to:
"A heralded photon waveform is equivalent to the two-photon correlation function. This correlation function is not symmetric in a cascade decay \cite{Loudon:2003}; a measurement for our system in shown in \cite{Srivathsan:2013}.
When the signal photon is used as herald, the time correlation shows a fast rise and a long exponential decay. We expect this behaviour to be time reversed when the idler serves as herald, with the exponential decay converted into an exponential rise.
We investigate the resulting temporal envelopes of the heralded single
photon fields by measuring the field quadrature in the time domain~\cite{Yuen:83,lvovsky:2001} via a balanced homodyne detection."



Here is a response to the criticisms of the referees:

Reply to Referee A

* We are glad that Referee A appreciates the changes to the original manuscript.
In our current revision we included her/his suggestion for a revised title, so that it now reads:
"Generation of an exponentially rising single photon field from parametric conversion in atoms".
We believe that this title reflects the original spirit of the manuscript without suggesting any misleading interpretations.


Reply to the Third Referee

* We are happy to see that the Third Referee appreciates the manuscript and we thank him for bringing to our attention the work of Brownell, Lu, and Hartmann, PRL 75, 3265 (1995).
We found it very interesting but not directly relevant for our case, in particular regarding the timing of the photon generation. In our previous article (ref.15 of the manuscript) we show a plot of the time correlation for detection events in the signal and idler modes. The curve is highly asymmetrical, supporting our interpretation of a time-ordered emission.

* Regarding the question of the coherence of the photon state, we appreciate the Referee suggestion to look into the works of Lvovsky's group.
It is indeed true that the intermediate level can undergo a decohering process and then decay spontaneously emitting a photon at the idler wavelength.
These emitted photons do not contribute to the phase matching of the parameteric process and, together with all the other possible decay channels, provides a fluorescence that is the main contribution to our background single photon count rate and that is independent of the direction of collection.
The coherence of the photon pair collected is instead ensured by the phase matching condition by an appropriate choice of the collection modes.

* We welcome the suggestion of a clarification of how we obtain an exponentially rising waveform. We have added the following clarifying text when introducing the single photon envelope measurement, and added a reference to a standard
textbook (Loudon) which a reader might find helpful:

"A heralded photon waveform is equivalent to the two-photon correlation function. This correlation function is not symmetric in a cascade decay \cite{Loudon:2003}; a measurement for our system in shown in \cite{Srivathsan:2013}.
When the signal photon is used as herald, the time correlation shows a fast rise and a long exponential decay. We expect this behaviour to be time reversed when the idler serves as herald, with the exponential decay converted into an exponential rise.
We investigate the resulting temporal envelopes of the heralded single
photon fields by measuring the field quadrature in the time domain~\cite{Yuen:83,lvovsky:2001} via a balanced homodyne detection."